diff --git a/packages/opencode/src/cli/cmd/run.ts b/packages/opencode/src/cli/cmd/run.ts
index 3927f615a080..9148f41b5da8 100644
--- a/packages/opencode/src/cli/cmd/run.ts
+++ b/packages/opencode/src/cli/cmd/run.ts
@@ -696,28 +696,40 @@ export const RunCommand = effectCmd({
         // created, and replies issued from inside the loop must use that client.
         async function loop(client: OpencodeClient, events: Awaited<ReturnType<typeof sdk.event.subscribe>>) {
           const toggles = new Map<string, boolean>()
+          // messageID -> model that produced it, so step events can carry the
+          // attribution that only lives on the assistant message. Only filled from
+          // messages seen on this stream, so a step for a message created before
+          // this subscription (attaching to a turn already in flight) emits
+          // without the fields rather than guessing.
+          const models = new Map<string, { providerID: string; modelID: string }>()
           let error: string | undefined
 
           for await (const event of events.stream) {
             if (
               event.type === "message.updated" &&
               event.properties.sessionID === sessionID &&
-              event.properties.info.role === "assistant" &&
-              args.format !== "json" &&
-              toggles.get("start") !== true
+              event.properties.info.role === "assistant"
             ) {
-              UI.empty()
-              UI.println(`> ${event.properties.info.agent} · ${event.properties.info.modelID}`)
-              UI.empty()
-              toggles.set("start", true)
+              models.set(event.properties.info.id, {
+                providerID: event.properties.info.providerID,
+                modelID: event.properties.info.modelID,
+              })
+              if (args.format !== "json" && toggles.get("start") !== true) {
+                UI.empty()
+                UI.println(`> ${event.properties.info.agent} · ${event.properties.info.modelID}`)
+                UI.empty()
+                toggles.set("start", true)
+              }
             }
 
             if (event.type === "message.part.updated") {
               const part = event.properties.part
               if (part.sessionID !== sessionID) continue
 
+              const model = models.get(part.messageID)
+
               if (part.type === "tool" && (part.state.status === "completed" || part.state.status === "error")) {
-                if (emit("tool_use", { part })) continue
+                if (emit("tool_use", { part, ...model })) continue
                 if (part.state.status === "completed") {
                   await tool(part)
                   continue
@@ -738,15 +750,15 @@ export const RunCommand = effectCmd({
               }
 
               if (part.type === "step-start") {
-                if (emit("step_start", { part })) continue
+                if (emit("step_start", { part, ...model })) continue
               }
 
               if (part.type === "step-finish") {
-                if (emit("step_finish", { part })) continue
+                if (emit("step_finish", { part, ...model })) continue
               }
 
               if (part.type === "text" && part.time?.end) {
-                if (emit("text", { part })) continue
+                if (emit("text", { part, ...model })) continue
                 const text = part.text.trim()
                 if (!text) continue
                 if (!process.stdout.isTTY) {
@@ -759,7 +771,7 @@ export const RunCommand = effectCmd({
               }
 
               if (part.type === "reasoning" && part.time?.end && thinking) {
-                if (emit("reasoning", { part })) continue
+                if (emit("reasoning", { part, ...model })) continue
                 const text = part.text.trim()
                 if (!text) continue
                 const line = `Thinking: ${text}`
diff --git a/packages/opencode/test/cli/run/run-process.test.ts b/packages/opencode/test/cli/run/run-process.test.ts
index bd5847e2723c..54d47d617eb6 100644
--- a/packages/opencode/test/cli/run/run-process.test.ts
+++ b/packages/opencode/test/cli/run/run-process.test.ts
@@ -6,7 +6,7 @@
 import { describe, expect } from "bun:test"
 import { Effect } from "effect"
 import { reply } from "../../lib/llm-server"
-import { cliIt } from "../../lib/cli-process"
+import { cliIt, testModelID } from "../../lib/cli-process"
 
 describe("opencode run (non-interactive subprocess)", () => {
   // Happy path: prompt completes, output reaches stdout, process exits 0.
@@ -121,13 +121,26 @@ describe("opencode run (non-interactive subprocess)", () => {
           expect(typeof evt.sessionID).toBe("string")
         }
         expect(events.map((event) => event.type)).toEqual(["step_start", "text", "step_finish"])
+        const [provider, model] = testModelID.split("/")
         expect(events.map(({ timestamp: _, sessionID: __, ...event }) => event)).toEqual([
-          { type: "step_start", part: expect.objectContaining({ type: "step-start" }) },
+          {
+            type: "step_start",
+            part: expect.objectContaining({ type: "step-start" }),
+            providerID: provider,
+            modelID: model,
+          },
           {
             type: "text",
             part: expect.objectContaining({ type: "text", text: "structured output" }),
+            providerID: provider,
+            modelID: model,
+          },
+          {
+            type: "step_finish",
+            part: expect.objectContaining({ type: "step-finish" }),
+            providerID: provider,
+            modelID: model,
           },
-          { type: "step_finish", part: expect.objectContaining({ type: "step-finish" }) },
         ])
         expect(result.stdout.endsWith("\n")).toBe(true)
         expect(
@@ -140,6 +153,62 @@ describe("opencode run (non-interactive subprocess)", () => {
     60_000,
   )
 
+  // The model only exists on the assistant message, so the events have to pick it
+  // up from there. Running a non-default model proves the attribution follows the
+  // model that produced the part instead of a fixed value, and a tool call makes
+  // the run span several part types.
+  cliIt.concurrent(
+    "--format json attributes every part-bearing event to the model that produced it",
+    ({ llm, opencode }) =>
+      Effect.gen(function* () {
+        yield* llm.push(
+          reply().reason("thinking it over").text("before tool").tool("bash", {
+            command: "printf tool-output",
+            description: "Print deterministic output",
+          }),
+        )
+        yield* llm.text("after tool")
+        const result = yield* opencode.run("say hi", {
+          format: "json",
+          model: "test/test-model-alt",
+          extraArgs: ["--thinking"],
+        })
+        opencode.expectExit(result, 0)
+
+        const events = opencode.parseJsonEvents(result.stdout)
+        expect(new Set(events.map((event) => event.type))).toEqual(
+          new Set(["step_start", "reasoning", "text", "tool_use", "step_finish"]),
+        )
+        for (const event of events) {
+          expect(event.providerID).toBe("test")
+          expect(event.modelID).toBe("test-model-alt")
+        }
+      }),
+    60_000,
+  )
+
+  // Every other test passes --model, so they can't tell the model being read off
+  // the message from one read off the argv. Without --model the server resolves
+  // the configured default and the argv holds nothing, so this run only reports
+  // a model if it really comes from the assistant message.
+  cliIt.concurrent(
+    "--format json attributes events to the resolved default model without --model",
+    ({ llm, opencode }) =>
+      Effect.gen(function* () {
+        yield* llm.text("default model run")
+        const result = yield* opencode.spawn(["run", "--format", "json", "say hi"])
+        opencode.expectExit(result, 0)
+
+        const events = opencode.parseJsonEvents(result.stdout)
+        expect(events.length).toBeGreaterThan(0)
+        for (const event of events) {
+          expect(event.providerID).toBe("test")
+          expect(event.modelID).toBe("test-model")
+        }
+      }),
+    60_000,
+  )
+
   cliIt.concurrent(
     "--format json emits a pure error record for a rejected prompt request",
     ({ opencode }) =>
diff --git a/packages/opencode/test/lib/test-provider.ts b/packages/opencode/test/lib/test-provider.ts
index cfb5a93e33e5..f30c8c20cc96 100644
--- a/packages/opencode/test/lib/test-provider.ts
+++ b/packages/opencode/test/lib/test-provider.ts
@@ -1,7 +1,8 @@
 // Shared provider config for tests that need opencode to talk to a fake LLM
-// over a real HTTP endpoint. Registers a single provider `test` with a single
-// model `test-model` (i.e. `--model test/test-model`), pointed at the URL the
-// caller supplies (typically a TestLLMServer instance).
+// over a real HTTP endpoint. Registers a single provider `test` with two models,
+// `test-model` (the default `--model test/test-model`) and `test-model-alt`, for
+// tests that need output attributed to a specific model. Both point at the URL
+// the caller supplies (typically a TestLLMServer instance).
 //
 // Used by:
 //   - test/lib/run-process.ts          (subprocess CLI tests)
@@ -10,6 +11,9 @@ export function testProviderConfig(llmUrl: string) {
   return {
     formatter: false,
     lsp: false,
+    // Default model, so a run without --model still resolves one and tests can
+    // tell a value read off the message from one read off the argv.
+    model: "test/test-model",
     provider: {
       test: {
         name: "Test",
@@ -29,6 +33,20 @@ export function testProviderConfig(llmUrl: string) {
             cost: { input: 0, output: 0 },
             options: {},
           },
+          // Second model so tests can assert output is attributed to the model
+          // that produced it, not to a hardcoded default.
+          "test-model-alt": {
+            id: "test-model-alt",
+            name: "Test Model Alt",
+            attachment: false,
+            reasoning: false,
+            temperature: false,
+            tool_call: true,
+            release_date: "2025-01-01",
+            limit: { context: 100_000, output: 10_000 },
+            cost: { input: 0, output: 0 },
+            options: {},
+          },
         },
         options: { apiKey: "test-key", baseURL: llmUrl },
       },